Conventionally, as image heating devices typified by thermofixing devices, contact heating type devices such as of a roller heating type and a belt heating type have been in general use.
In recent years, in response to the demand for a reduction in power consumption and warm-up time, roller heating type and belt heating type devices employing an electromagnetic induction heating method have been proposed.
FIG. 20 shows an example of a conventional image heating device including a heating roller that is heated by electromagnetic induction (see, for example, JP11(1999)-288190 A).
In FIG. 20, reference numeral 820 denotes a heating roller including a supporting layer 824 made of metal, an elastic layer 823 that is formed from a heat-resistant foam rubber and molded integrally on an outer surface of the supporting layer 824, a heat generating layer 821 formed of a metallic tube, and a mold releasing layer 822 provided on an outer surface of the heat generating layer 821, which are provided outwardly in this order. Reference numeral 827 denotes a pressing roller that is formed from a heat-resistant resin and has the shape of a hollow cylinder. A ferrite core 826 wound with an excitation coil 825 is placed in an inner portion of the pressing roller 827. The ferrite core 826 applies pressure to the heating roller 820 through the pressing roller 827, and thus a nip part 829 is formed. While the heating roller 820 and the pressing roller 827 rotate in the respective directions indicated by arrows, a high-frequency current is fed through the excitation coil 825. This causes alternating magnetic fields H to be generated, so that the heat generating layer 821 of the heating roller 820 is heated rapidly by electromagnetic induction to a predetermined temperature. While predetermined heating is continued in this state, a recording material 840 is inserted into and passed through the nip part 829. Thus, toner images 842 formed on the recording material 840 are fixed on the recording material 840.
Furthermore, in addition to devices of the above-mentioned roller heating type using the heating roller 820 having the induction heat generating layer 821 as shown in FIG. 20, devices of the belt heating type using an endless belt including an induction heat generating layer have been proposed. FIG. 21 shows an example of a conventional image heating device using an endless heating belt that is heated by electromagnetic induction (see, for example, JP10(1998)-74007 A).
In FIG. 21, reference numeral 960 denotes a coil assembly as an excitation unit that generates a high-frequency magnetic field. Reference numeral 910 denotes a metal sleeve (heating belt) that generates heat under a high-frequency magnetic field generated by the coil assembly 960. The metal sleeve 910 is formed by coating a surface of an endless tube made from a thin layer of nickel or stainless with a fluorocarbon resin. An inner pressing roller 920 is inserted in an inner portion of the metal sleeve 910, and an outer pressing roller 930 is placed outside the metal sleeve 910. The outer pressing roller 930 is pressed against the inner pressing roller 920 such that the metal sleeve 910 is interposed between them, and thus a nip part 950 is formed. While the metal sleeve 910, the inner pressing roller 920, and the outer pressing roller 930 rotate in the respective directions indicated by arrows, a high-frequency current is fed through the coil assembly 960. Thus, the metal sleeve 910 is heated rapidly by electromagnetic induction to a predetermined temperature. While predetermined heating is continued in this state, a recording material 940 is inserted into and passed through the nip part 950. Thus, a toner image formed on the recording material 940 is fixed on the recording material 940.
In each of the image heating devices employing the electromagnetic induction heating method, which are shown in FIGS. 20 and 21, a further reduction in warm-up time requires a reduction in thermal capacity of the heat generating layer heated by induction heating, i.e. a reduction in thickness of the heat generating layer.
However, in the image heating device of the roller heating type shown in FIG. 20, in order to obtain a desired thermal capacity by reducing a thickness of the heat generating layer 821 while using an electric current at the same frequency as an electric current to be applied to the excitation coil 825, it is required that the thickness be reduced so as to be smaller than a skin depth, i.e. a thickness defined by a flow of an induction current. With such a reduction in thickness, magnetic flux (leakage magnetic flux) that penetrates the heat generating layer 821 so as to leak therefrom is increased, so that in the supporting layer 824, an eddy current is generated to cause the supporting layer 824 to be heated. As a result, for example, bearings supporting the supporting layer 824 are heated, and thus deterioration and breakage are caused in the bearings, and the rate of power contributing to heat generation of the heat generating layer 821 is decreased, thereby undesirably causing an increase in warm-up time, which have been disadvantageous.
Similarly, in the image heating device of the belt heating type shown in FIG. 21, in order to obtain a desired thermal capacity by reducing a thickness of a heat generating layer of the metal sleeve 910 while using an electric current at the same frequency as an electric current to be applied to the coil assembly 960, it is required that the thickness be reduced so as to be smaller than a skin depth, i.e. a thickness defined by a flow of an induction current. With such a reduction in thickness, magnetic flux that penetrates the heat generating layer to leak therefrom reaches the inner pressing roller 920, so that in the inner pressing roller 920, an eddy current is generated to cause the inner pressing roller 920 to be heated. As a result, for example, bearings supporting the inner pressing roller 920 are heated, and thus deterioration and breakage are caused in the bearings, and the rate of power contributing to heat generation of the heat generating layer is decreased, thereby undesirably causing an increase in warm-up time, which have been disadvantageous.
In order to prevent these problems, the skin depth should be reduced so as to be smaller than a thickness of the heat generating layer. However, in order to reduce the skin depth, it is required that an electric current at a higher frequency be applied, thereby resulting in problems such as an increase in cost of an excitation circuit and an increase in leaking electromagnetic wave noise.
Moreover, since the heat generating layer is deformed repeatedly at the nip part by the pressing roller (the pressing roller 827 shown in FIG. 20, the outer pressing roller 930 shown in FIG. 21), in the case of the heat generating layer formed by nickel electroforming, a problem of lower mechanical durability of the heat generating layer arises. Further, in the case of the heat generating layer formed from stainless steel, while improved durability is provided, a problem of an increase in warm-up time arises.